Update buffer swap test to use better variable names

[neeharv]

tests/parallel/buffer-swap.js has all variables defined at the top level scope, and hence the variable names are variations of buf, buf2, buf3 etc.

This PR puts each setup/assertion code block into its own block, and renames the variables to be easier to read.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

[neeharv]

cc @Trott

[Fishrock123]

CI: https://ci.nodejs.org/job/node-test-pull-request/7554/

[cjihrig]

Isn't the point of moving to block scoping to avoid shared variables like this?

[mscdex]

How about either moving the commonBuf usage at the bottom of the file into the area where commonBuf is set or move the code where commonBuf is set to the bottom of the file, remove its surrounding braces, and rename commonBuf to buf or something ('commonBuf' does not seem like a good name/description).

[neeharv]

@mscdex @cjihrig I wasn't sure of moving the test code around too much, but what you're saying makes sense. I've moved the length assertions into the block where the relevant buffer variable was being defined.

[neeharv]

Not sure why CI failed. All tests pass fine locally (OS X), looks like the fedora build failing was because I force pushed some changes while the tests were running.

[mscdex]

CI again: https://ci.nodejs.org/job/node-test-pull-request/7562/

[Trott]

Not sure why CI failed.

The two CI failures in the previous run look like build failures unrelated to this test change. Hopefully the newer CI run will come back all green.

[jasnell]

Landed in 5096651